This invention relates to tangentially fired pulverized coal furnaces and in particular to a fuel and air supply means therefore.
Tangentially coal fired furnaces include fuel nozzles which introduce a mixture of primary air and coal from the corners toward an imaginary circle in the center of the furnace. Secondary air is distributed around the introduced coal stream and in intermediate adjacent streams. The coal nozzle does not operate as an independent burner but rather introduces the coal to the swirling mass in the center of the furnace which is in fact the burner in this type furnace. Accordingly, precise control of air and coal to each of the nozzle locations is not required. Furthermore, the long turbulent flame provides time for cooling of the gases while combustion is still continuing thereby resulting in lower peak flame temperatures and a lower production of oxides of nitrogen as compared to alternative burning methods.
At high ratings, there exists a strong interchange of energy between flames eminating fom the various sources. It is this exchange that provides the energy for the volitization and subsequent ignition of incoming coal. As load is decreased this interchange of energy weakens, and at some point supplemental fuel injected in close proximity to the coal stream, must provide additional energy to augment the normal interchange.
At extremely low ratings there is a minimum furnace air requirement which demands substantial air flow beyond that required to burn the fuel. This results in a chilling effect on the combustion process, thereby aggravating the smoking tendency.
Once a parallel flow pattern leaving the nozzles is established, it is difficult to change this flow pattern with coal as compared to other fuels. The coal and transport or "primary" air itself have a larger weight mass with respect to the remainder or "secondary" air required to burn it as compared to oil or gas. The use of primary air at that location also decreases the amount of secondary air available to deflect the coal stream. Operations which are successful for manipulating an oil flame, therefore, will not necessarily be adequate when pulverized coal is the fuel.